1. Field of the Invention
The invention relates to optical measurement systems and, more particularly, to a technique for empirically determining the delay characteristics of an optical receiver, storing those characteristics as digital data, and using the data to effect phase compensation in a measurement process.
2. Description of the Related Art
Laser interferometry systems are designed to measure distance and, derivatively, motion with exceptional accuracy. A typical laser interferometer architecture uses doppler-shifted laser signals in such a way that the phase difference between a reference signal and a measured signal is equivalent to an absolute position, minus a fixed offset. Consequently, any non-static delay in the measurement causes an error in the position data reported by the system. Compensation for a small constant delay in the measurement may be achieved with known methods, but an uncompensated delay change with respect to input frequency, input signal power, or system temperature will cause a position error. If the quantitative relationship between phase delay and any of these parameters can be measured during final product testing of the measurement system, compensation may be possible.
In a laser interferometer system, the device that converts the optical signal from the laser to an electrical signal is referred to as a xe2x80x9creceiver.xe2x80x9d This electrical signal is then sent to a digital phase measurement device, called an xe2x80x9caxis cardxe2x80x9d or xe2x80x9cphase measurement card.xe2x80x9d An axis card may have multiple channels, or axes, of measurement.
Heretofore, methods for compensating for non-static receiver delay have used trimpot-based analog circuits in order to cancel first-order phase errors of the receiver circuitry. The trimpots are adjusted by a technician in final production test Although useful for compensating for phase errors that result from latitude in component tolerances, this technique requires an excessive number of trimpots to compensate for all sources of phase error.
An additional source of error in interferometric position measurements originates with the differences in the fixed delays between the measured signal path and reference signal path. The sources of the fixed delays are numerous and include, for example: cable lengths, optical path lengths, photoelectric detector delay, circuit delay, and phase meter offset. The effects of these fixed delays constitute differences in the xe2x80x9cdata agexe2x80x9d of the measurement. That is, xe2x80x9cdata agexe2x80x9d delay derives from the elapsed time between the event representing the position measurement, and the availability of the position data to the user. Compensation for these fixed delays by adjusting one or more of the same fixed delays is generally impracticable. Compensation for these fixed delays in the prior art methods requires knowledge of the velocity of the object whose position is being measured, as well as the delay in each measured axis. An approach to this source of measurement error is described in U.S. Pat. No. 5,767,972, Method and Apparatus for Providing Data Age Compensation in an Interferometer.
In addition, those skilled in the art of laser measurement techniques recognize that the delay that is attributable to the receiver itself is a function of a number of operating parameters, including, inter alia, input frequency, signal power, and temperature, due to the nature of the receiver electronics. Furthermore, the specific manner in which receiver delay varies with applicable operating parameters varies from receiver to receiver.
Accordingly, what is desired is a technique that enables a far greater range of non-static delay compensation than previous techniques using analog means. In particular, a technique that eliminates the need for analog trimpots would be welcome. Because phase delay may be expected to vary as a function of one or more operating parameters of the receiver, a significant performance improvement will be realized if the delay compensation can be made to track, or correlate to, variations in the relevant operating parameter(s). Also, to the extent that compensation data is presented in digital form, the data can be stored on the receiver, with little or no user interaction, thereby simplifying final test procedure. Also, it is common for optical measurement systems to be partitioned so that an optical receiver module is physically distinct and separable from an associated measurement module, thereby enabling the respective receiver and measurement modules to be separated and changed out for maintenance, repair or troubleshooting purposes. Consequently, system flexibility may be preserved if the delay compensation technique facilitates the interoperability of numerous receivers of the same kind with numerous measurement modules of the same kind.
What is needed and is disclosed herein is an invention that provides for a method and a system to determine and keep track of phase and time difference errors in an optical receiver and provide for proper compensation values to make up for such errors.
In an embodiment of the invention a first cycle of a measured signal of the optical receiver is measured. A reference receiver outputs a reference signal whose first cycle is measured. A reference or system clock is used to measure a time value of the next subsequent cycle of the measured signal to the next pulse of the system clock, and the next subsequent cycle of the reference signal to the next pulse of the reference clock. The time period of the measured signal to the next pulse of the system clock is divided by the cycle of the measured signal, and the time period of the reference signal to the next pulse of the system clock is divided by the cycle of the reference signal. The difference between the two quotients calculated from the respective division calculation is correlated as phase error.
In certain embodiments of the invention the quotient derived from the measured signal is calculated as a first derivative over time to determine a frequency value. The subsequent frequency value can be correlated with the phase error in a lookup table or similar memory application.
In other embodiments of invention, a second phase error is calculated by calculating a second position value. The second position value is derived by calculating a second set of time values measured respectively from the next subsequent cycle of the measured signal, and the next subsequent cycle of the reference signal to the clock pulse following the next subsequent clock pulse. The second set of values are divided by the respective cycle values. The difference of resulting values are derived and treated as position or phase error.
In yet other embodiments of the invention time difference errors are calculated by taking the time of a measured signal to subsequent clock pulse, taking the time of a reference signal to a subsequent clock pulse, and calculating the difference between the two times. The time difference can be associated with operating parameters such as a frequency and stored in a lookup table or similar memory application.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.